Radio frequency emitters (radars, satellite uplink stations, cell-phone base stations, relay links) can be detected, analysed, and geo-referenced from a remote observation platform. This is achieved using a sensor with an antenna system for detecting the radiation, connected to a receiver and processing system. These systems can be deployed from satellites, aircraft, UAVs, ships vehicles or mounted in masts.
Typical solutions employ radio receiver systems operating in the frequency bands 1 through 12 GHz. These systems employ multiple receiving antennas and multiple receivers to derive a course direction to the emitters.
In one method, the direction to the emitters is determined by comparing the phases of signals received in two or more antenna panels. However, it is well known that that the phase angle determined when comparing the signals from two receiving antennas will repeat itself at even intervals. This means that a given phase angle will not be conclusive as regards the direction to the emitter. To solve this ambiguity, prior art systems have included an omni-directional guard antenna in addition to the directive search antennas. The amplitude of the signal received by the guard antenna is compared with the signal amplitude from one of the search antennas in order to decide which angular direction in space that corresponds with a given phase angle.
Stacking by combining the signals from a number of element antennas are common in order to obtain an increase in antenna gain, to narrow the main antenna lobe or steer the direction of the main lobe. The increase in gain is a result of the narrowing of the lobe width. However, combining signals from individual element antennas tend to introduce some problems: The element antennas will easily interfere with each other and create an overall antenna diagram with so called grating lobes, which are side lobes with gain figures only slightly lower than in the main lobe. Grating lobes may be eliminated by using a very small distance between adjacent element antennas, but at the cost of a lower gain for a given number of element antennas. The signals from the individual element antennas are combined in combiners/splitters that introduce losses. A solution to this is to use separate access (amplifiers) to/from each element antenna, but at an increase in hardware costs. Thus, stacking of antennas introduces conflicting requirements between the desires of obtaining as large a gain as possible, obtaining a clean direction diagram and avoiding excessive hardware costs.